It is desirable to provide low current drain wireless communication means to facilitate information transfer, particularly in battery powered devices. Alternating current (AC) coupled, direct detection receivers are employed to implement optical wireless communication. Such receivers are very attractive in low current wireless data communications due to their simplicity and low cost. However, full swing noise present at the output of these receivers in the absence of received data impedes their utility.
In systems that employ a serial data format, the receiver may translate noise into false characters. In order for most systems to operate properly under noisy conditions, the danger of translating noise into data at the output of the receiver must be eliminated. There are two known methods of achieving this. One method uses hardware to "squelch" the receiver output in the presence of noise using a gate at the output of the receiver. Another method employs a software protocol to desense the system to noise.
A difficulty encountered when implementing a squelch circuit is avoiding a reduction in receiver sensitivity while gating the receiver output. By giving the squelch enough margin (such as hysterisis) to distinguish between data and noise, and to avoid "chattering" at weak signal levels, receiver sensitivity is reduced by selecting a compromise level at which squelching occurs. In low current systems, this degradation in the receiver sensitivity can not be accommodated, as no compromise level is available at which squelching can be provided without also removing data. Another problem encountered is accommodating the complexity of the squelch circuitry suitable for wireless infrared communications. In order for the squelch circuit to operate properly, it must be insensitive to the various light sources which are the main source of interference in wireless IR systems. Designing a robust squelch circuit to satisfy these requirements can be difficult and adds significantly to the cost and current demand of an otherwise simple, inexpensive, and low current receiver design.
A second method of preventing the translation of noise to data relies on a software protocol. This approach does not compromise the receiver sensitivity and avoids the additional cost and current demand of squelch circuits. Known protocols overcome the effects of noise by looking for specific data format. If random noise is encountered, the protocol is capable of recognizing the presence of such noise and responding to it accordingly. However known data formats are not well suited to implementation in low power, battery operated devices. The InfraRed Data Association (IrDA) serial communications link uses a Universal Asynchronous Receiver/Transmitter (UART) device. However, the IrDA specification requires additional circuitry to generate the IrDA signal format compatible with the UART device. Further, the IrDA specification requires that the receiving apparatus continuously monitor the infrared link for incoming data.